Copyright 2008 Society of Photo-Optical Instrumentation Engineers. This paper was published in SPIE volume 6799: Modelling and THZ Technology and is made available as an electronic reprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.